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Hubble Space Telescope
COS Spectroscopic Observing Strategies

Time Resolution

Time-resolved observations can be made either in ACCUM mode or in TIME-TAG mode. However TIME-TAG is the preferred mode because it provides significant post-pipeline advantages for temporal sampling, exclusion of poor quality data, and, for the FUV, improved thermal correction and better background removal (by using the pulse-height information). At present, TIME-TAG should not be used for count-rates greater than 30,000 counts-sec–1. ACCUM mode should be used only when absolutely necessary, such as for high count-rate targets. For TIME-TAG mode, one must choose the BUFFER-TIME to control the frequency of data transfer.

Signal-to-noise Considerations

In ground testing, the COS FUV channel was capable of routinely delivering fully reduced spectra with a photon-noise-limited signal-to-noise (S/N) ratio of ~18 per resolution element in a single exposure. Ground tests also show that the COS NUV MAMA can deliver S/N up to about 50 without using a flat field, just based on photon statistics.

In order to achieve higher S/N, COS can move the spectrum in small amounts so that it falls on different parts of the detector. For more information about the use of this FP-POS option see Section 5.8.2 of the COS Instrument Handbook.

Photometric (Flux) Precision

The limits on the precision and accuracy of fluxes measured with COS are expected to be the same as for STIS. COS has the advantage of a fairly large aperture so that there are only small aperture losses ( see Section 5.1.9 of the COS IHB). The photometric capabilities of COS will be tested after it is installed, but for now we take them to be the same as STIS, namely 5% accuracy on absolute fluxes and 2% on relative fluxes (within a single exposure). The experience with the NUV MAMA of STIS shows that the repeatability of a flux is good to well under 0.5%. The level of repeatability for the FUV detector is not yet known.

Spatial Resolution and Field of View

The spatial resolution of COS is inherently limited by the aberrated Point Spread Function of HST. Ground tests show that COS can separate spectra of two equally bright objects that are 1 arcsec apart in the cross-dispersion direction for either the FUV or NUV channel. The NUV channel’s optics can correct the aberrations so that the NUV imaging capability is diffraction limited (see Section 5.1.8 of the COS IHB). The field of view of COS is obviously determined by the entrance apertures that are 2.5 arcsec in diameter, but the aberrated light entering the aperture means that objects up to 2 arcsec from the center of the aperture will be visible in the recorded spectra

Wavelength Accuracy

The COS specifications for absolute wavelength uncertainties within an exposure are:
• 15 km s–1 for medium-resolution gratings,
• 150 km s–1 for G140L, and
• 175 km s–1 for G230L.
The error budget for wavelength accuracy for the various gratings then breaks down as shown in the table below. Wavelength calibration exposures will be routinely obtained with all science exposures (TAGFLASH and AUTO wavecals) and observers also may specify their own additional wavelength calibration exposures (GO wavecals).


Grating Error goal (1 σ) Internal Error (1 σ) External Error (1 σ) Plate Scale 1
km s-1 pixels pixels arcsec pixel arcsec-1
G130M 15 5.7 - 7.5 3.0 - 4.0 0.09 - 0.12 45.1
G160M 15 5.8 - 7.2 3.1 - 3.8 0.10 - 0.12 44.6
G140L 150 7.5 - 12.5 4.0 - 6.6 0.12 - 0.21 47.1
G185M 15 7.2 - 10.0 1.2 - 1.7 0.03 - 0.04 41.85
G225M 15 9.7 - 13.3 1.6 - 2.3 0.04 - 0.06 41.89
G285M 15 9.7 - 14.7 1.6 - 2.6 0.05 - 0.07 41.80
G230L 175 8.3 - 15.5 1.4 - 2.6 0.03 - 0.07 42.27
  1. The plate scale is shown to indicate the centering precision required during acquisition. These pre-launch values, determined from ray traces, are for the along-dispersion direction.

Spectroscopic Resolving Power

The available spectroscopic resolving powers (R) available to observers with COS are listed in Table 5.1 of the COS Instrument Handbook. Note that no single value of R applies to any one grating, instead it depends on wavelength, with R ∝ λ. The Resolving power also depends on the position of the source in the COS aperture. Use of the BOA leads to a degradation of R by factors of 3 to 5.

Sensitivity to second-order spectra

COS has been designed to avoid contamination of the first-order spectra by any second-order light. The result from ground tests is that only G225M shows measurable second-order throughput, but even then the second-order light was suppressed by factors of 3,000 to 10,000.